Variations from the ideal peak form can be described by a so-called asymmetry factor T. This is defined as the ratio of the distances A and B between the central verticals and the slopes of the distribution at 10% of their height. Preferable values of T: 0.8 < T < 1.3. Values above T = 2 should be avoided.
Terminology:
T < 1 is Fronting
T > 1 is Tailing.
Graphic